The present invention relates generally to communication systems and, more particularly, to an iterative system and method for optimizing traffic load distribution in a Code Division Multiple Access (CDMA) based wireless network.
The function of assigning each mobile station in a network to a set of CDMA base station sectors is termed xe2x80x9cCDMA cell loadingxe2x80x9d or xe2x80x9cCDMA traffic load distribution.xe2x80x9d The uneven spatial distribution (i.e., the random location of wireless devices) and density of CDMA offered traffic loads, as well as non-uniform terrain and other factors, make the assignment of each wireless device in the network to the sectors that are closest to its location inefficient and sub-optimal.
Several theoretical attempts have been made to address the problem of optimal load distribution in order to maximize the capacity of CDMA cellular networks. Two such theoretical attempts have been described in S. Hanly, xe2x80x9cAn Algorithm for Combined Cell-Site Selection and Power Control to Maximize Cellular Spread Spectrum Capacity,xe2x80x9d IEEE Journal on Selected Areas in Communications, Vol. 13, No. 7, September 1995, and R. Yates et al., xe2x80x9cIntegrated Power Control and Base Station Assignment,xe2x80x9d IEEE Transactions on Vehicular Technology, Vol. 44, No. 3, August 1995. The implementation of the theoretical algorithms described in these documents in a practical system is extremely difficult since it is assumed that each wireless device has continuous information regarding the path loss between its location and all surrounding CDMA sectors (reverse link), as well as on the interference level experienced by all these sectors.
In order to address this problem in practical IS-95-based CDMA networks, each wireless device is assigned to a set of base stations (best CDMA servers) based on forward link received pilot power over interference (Ec/Io) measurements at the location of the wireless device. In order to optimize cell loading and, therefore, the CDMA capacity of an IS-95-based system, a network designer should set the pilot powers of all CDMA sectors at levels such that the resulting traffic distribution based on forward link Ec/Io measurements would be the same as the theoretical distribution based on reverse link information.
Using existing methods, a network designer would approximate the cell loading optimization problem by selecting an initial set of pilot transmit powers and performing a full CDMA simulation. The network designer then assesses the results of the simulation and a new set of pilot power settings would be introduced, followed by a new full CDMA simulation. This process would be repeated several times in a trial and error manner until satisfactory pilot power settings are derived. One skilled in the art would appreciate, however, that full-scale CDMA simulations, due to their complexity and accuracy, generally take several hours or days to get results. Therefore, the above-described trial and error adjustments may lead to a considerable amount of time being wasted analyzing network parameters that may not be useful.
As a result, there exists a need for a system and method that quickly determine an optimal load distribution and the pilot power levels needed to achieve the distribution.
Systems and methods consistent with the present invention address this need by providing an iterative process that determines an optimal load distribution in a communication network as well as the pilot power levels necessary to achieve the optimal load distribution.
A system consistent with the present invention determines the reverse interference level at each sector in the network and an average reverse interference level for the network. The system then adjusts the pilot power level of each sector by an amount equal to the difference between a sector""s reverse interference level and the network""s average reverse interference level. The system then assigns, based on the adjusted pilot power levels, each wireless device in the network to a sector.